You have 20 turns to get 5000 gold and kill the enemy leader. It's important to keep your army healthy and stop to rest for a turn or two on the way to the boss. Start out by taking the 5 or 6 enemies that approach your force of 5, position the melee in front, archers in back. Your meele fighters will end up a bit beat up but that's ok as long as the archers don't get hit. Send a melee west to pick up the dwarven mobile stronghold at the Dwarves' Stronghold. Once Dwarnrock appears, let him tank the incoming riders then regather and rest your forces. Capture the closest mine then proceed north.

Directly north there's another mine which you need to capture, there's 2 ogres and an archer guarding it. Dwarnrock is a melee beast and is best to send in first. Once that's captured, send one unit east to capture the closest human mine. This will trigger a human eagle to attack, but you can just pick on it until it flies away. The human mage may also cast spells and summon a creature, but it will stop once the eagle flies away. Rest, cast Dwarnrocks AOE buffs and move on.

Next up is the north orc mine. There are stone trolls guarding the mine so soften it up with archers (you should have 4 of them) and the mobile stronghold then move in with Dwarnrock. This is the position you'll be making your stand against a large number of orcs including the boss. Keep your ranged units protected and form a wall - do not move too far south/southwest or ogres may come up from the southeast and really mess things up. I recruited two Dwarf Nobles at this point to help out. Once the boss is close enough send Dwarnrock in to deal some serious damage and finish him off. Once the boss is dead all you have to do is survive until you have 5000G. You may notice that eventually all the mines but the most recently claimed one become recaptured, but you should have plenty left to wait it out. I finished on turn 22 which I believe is 3 or 4 turns to spare.